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Why does INCREASING the outdoor temperatures on my cooling design days REDUCE my PTAC cooling coil autosized capacities?

asked 4 years ago

JustinLueker's avatar

updated 2 years ago

Two identical models, only difference is the SizingPeriod:DesignDay objects for heating design day, "Maximum Dry-Bulb Temperature" and "Wetbulb or DewPoint at Maximum DryBulb." In one model I've hard-inputted these cooling sizing temperatures to be about 4degC higher to approximate a 2050 climate scenario.

In the model with higher dry- and wet-bulb temperatures for my cooling design days, the HVACTemplate:Zone:PTAC cooling coils autosized to LOWER capacities by about 4%. That is, the autosized "Cooling Coil Gross Rated Total Capacity {W}."

This certainly goes against intuition... does the model take into account other factors when autosizing PTAC cooling coils? What am I missing?

Model 1 cooling design day:

SizingPeriod:DesignDay,
Boston Logan IntL Arpt Ann Clg .4% Condns DB=>MWB,  !- Name
7,                       !- Month
21,                      !- Day of Month
SummerDesignDay,         !- Day Type
32.7,                    !- Maximum Dry-Bulb Temperature {C}
8.4,                     !- Daily Dry-Bulb Temperature Range {deltaC}
,                        !- Dry-Bulb Temperature Range Modifier Type
,                        !- Dry-Bulb Temperature Range Modifier Day Schedule Name
Wetbulb,                 !- Humidity Condition Type
22.9,                    !- Wetbulb or DewPoint at Maximum Dry-Bulb {C}
,                        !- Humidity Condition Day Schedule Name
,                        !- Humidity Ratio at Maximum Dry-Bulb {kgWater/kgDryAir}
,                        !- Enthalpy at Maximum Dry-Bulb {J/kg}
,                        !- Daily Wet-Bulb Temperature Range {deltaC}
101253,                  !- Barometric Pressure {Pa}
6.3,                     !- Wind Speed {m/s}
270,                     !- Wind Direction {deg}
No,                      !- Rain Indicator
No,                      !- Snow Indicator
No,                      !- Daylight Saving Time Indicator
ASHRAEClearSky,          !- Solar Model Indicator
,                        !- Beam Solar Day Schedule Name
,                        !- Diffuse Solar Day Schedule Name
,                        !- ASHRAE Clear Sky Optical Depth for Beam Irradiance (taub) {dimensionless}
,                        !- ASHRAE Clear Sky Optical Depth for Diffuse Irradiance (taud) {dimensionless}
1;                       !- Sky Clearness

Model 2 cooling design day:

SizingPeriod:DesignDay,
Boston Logan IntL Arpt Ann Clg .4% Condns DB=>MWB,  !- Name
7,                       !- Month
21,                      !- Day of Month
SummerDesignDay,         !- Day Type
36.63,                   !- Maximum Dry-Bulb Temperature {C}
8.4,                     !- Daily Dry-Bulb Temperature Range {deltaC}
,                        !- Dry-Bulb Temperature Range Modifier Type
,                        !- Dry-Bulb Temperature Range Modifier Day Schedule Name
Wetbulb,                 !- Humidity Condition Type
26.12,                   !- Wetbulb or DewPoint at Maximum Dry-Bulb {C}
,                        !- Humidity Condition Day Schedule Name
,                        !- Humidity Ratio at Maximum Dry-Bulb {kgWater/kgDryAir}
,                        !- Enthalpy at Maximum Dry-Bulb {J/kg}
,                        !- Daily Wet-Bulb Temperature Range {deltaC}
101253,                  !- Barometric Pressure {Pa}
6.3,                     !- Wind Speed {m/s}
270,                     !- Wind Direction {deg}
No,                      !- Rain Indicator
No,                      !- Snow Indicator
No,                      !- Daylight Saving Time Indicator
ASHRAEClearSky,          !- Solar Model Indicator
,                        !- Beam Solar Day Schedule Name
,                        !- Diffuse Solar Day Schedule Name
,                        !- ASHRAE Clear Sky Optical Depth for Beam Irradiance (taub) {dimensionless}
,                        !- ASHRAE Clear Sky Optical Depth for Diffuse Irradiance (taud) {dimensionless}
1;                       !- Sky Clearness

Both models are set for system sizing:

SimulationControl,
Yes,                     !- Do Zone Sizing Calculation
Yes,                     !- Do System Sizing Calculation
Yes,                     !- Do Plant Sizing Calculation
Yes,                     !- Run Simulation for Sizing Periods
Yes,                     !- Run Simulation for Weather File Run Periods
Yes;                     !- Do HVAC Sizing Simulation for Sizing Periods

Thanks all!

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Comments

Have you compared the ERR files between the two runs? During sizing, EnergyPlus might be adjusting parameters like sensible heat ratio if they go outside of expected ranges.

Aaron Boranian's avatar Aaron Boranian  ( 4 years ago )

1 Answer

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1

answered 4 years ago

khaddad's avatar

The moisture content of the air of the updated case could be lower. This then will result in a smaller latent load and therefore lower total cooling load.

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Asked: 4 years ago

Seen: 185 times

Last updated: Jul 20 '20